Posts Tagged ‘sas vsa’

In my last post, I mentioned a much complained about “idle” CPU utilization quirk with NexentaStor when running as a virtual machine. After reading many supposed remedies on forum postings (some reference in the last blog, none worked) I went pit-bull on the problem… and got lucky.

For me, the problem cropped-up while running storage benchmarks on some virtual storage appliances for a client. These VSA’s are bound to a dedicated LSI 9211-8i SAS/6G controller using VMware’s PCI pass-through (Host Configuration, Hardware, Advanced Settings). The VSA uses the LSI controller to access SAS/6G disks and SSDs in a connected JBOD – this approach allows for many permutations on storage HA and avoids physical RDMs and VMDKs. Using a JBOD allows for attachments to PCIe-equipped blades, dense rack servers, etc. and has little impact on VM CPU utilization (in theory).

So I was very surprised to find idle CPU utilization (according to ESXi’s performance charting) hovering around 50% from a fresh installation. This runs contrary to my experience with NexentaStor, but I’ve seen reports of such issues on the forums and even on my own blog. I’ve never been able to reproduce more than a 15-20% per vCPU bias between what’s reported in the VM and what ESXi/vCenter sees. I’ve always attributed this difference to vSMP and virtual hardware (disk activity) which is not seen by the OS but is handled by the VMM.

CPU record of idle and IOzone testing of SAS-attached VSA

During the testing phase, I’m primarily looking at the disk throughput, but I notice a persistent CPU utilization of 50% – even when idle. Regardless, the 4 vCPU VSA appears to perform well (about 725MB/sec 2-process throughput on initial write) despite the CPU deficit (3 vCPU test pictured above, about 600MB/sec write). However, after writing my last blog entry, the 50% CPU leach just kept bothering me.

After wasting several hours researching and tweaking with very little (positive) effect, a client e-mail prompted a NMV walk through with resulted in an unexpected consequence: the act of powering-off the VSA from web GUI (NMV) resulted is significantly reduced idle CPU utilization.

Getting lucky: noticing a trend after using NMV to reboot for a client walk-through of the GUI.

Working with the 3 vCPU VSA over the previous several hours, I had consistently used the NMC (CLI) to reboot and power-off the VM. The fact of simply using the NMV to shutdown the VSA couldn’t have anything to do with idle CPU consumption, could it? Remembering that these were fresh installations I wondered if this was specific to a fresh installation or could it show up in an upgrade. According to the forums, this only hampered VMs, not hardware.

I grabbed a NexentaStor 3.1.0 VM out of the library (one that had been progressively upgraded from 3.0.1) and set about the upgrade process. The result was unexpected: no difference in idle CPU from the previous version; this problem was NOT specific to 3.1.2, but specific to the installation/setup process itself (at least that was the prevailing hypothesis.)

Regression into my legacy VSA library, upgrading from 3.1.1 to 3.1.2 to check if the problem follows the NexentaStor version.

If anything, the upgraded VSA exhibited slightly less idle CPU utilization than its previous version. Noteworthy, however, was the extremely high CPU utilization as the VSA sat waiting for a yes/no response (NMC/CLI) to the “would you like to reboot now” question at the end of the upgrade process (see chart above). Once “no” was selected, CPU dropped immediately to normal levels.

Now it seemed apparent that perhaps an vestige of the web-based setup process (completed by a series of “wizard” pages) must be lingering around (much like the yes/no CPU glutton.) Fortunately, I had another freshly installed VSA to test with – exactly configured and processed as the first one. I fired-up the NMV and shutdown the VSA…

Confirming the impact of the "fix" on a second fresh installed NexentaStor VSA

After powering-on the VM from the vSphere Client it was obvious. This VSA had been running idle for some time, so it’s idle performance baseline – established prior across several reboots from CLI – was well recorded by the ESXi host (see above.) The resulting drop in idle CPU was nothing short of astounding: the 3 vCPU configuration has dropped from a 50% average utilization to 23% idle utilization. Naturally, these findings (still anecdotal) have been forwarded on to engineers at Nexenta. Unfortunately, now I have to go back and re-run my storage benchmarks; hopefully clearing the underlying bug has reduced the needed vCPU count…

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SOLORI's Take and Quick Take posts express my personal opinion unless explicitly attributed to other sources. Where possible, supporting facts are presented to properly frame and ground these opinions, however they are presented "AS-IS" without regard to warranty or promise: expressed or implied.

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